Ceramic oxide compounds

ABSTRACT

A La 1-x  Sr x  Cr 1-y  Mn y  O 3  ceramic oxide gel obtainable by the process comprising the steps of: 
     (a) providing a colloidal solution of polyhydroxy organic chelating agents selected from the group consisting of ethylene glycol and citric acid and metal salts selected from the group consisting of lanthanum, strontium, chromium, and manganese, each salt being present in an amount necessary to provide the stoichiometric amount of metal ions required in the ceramic oxide gel; 
     (b) heating the colloidal solution to hydrolyze and polymerize the metal ions in the solution to a mixed metal oxide precursor of the ceramic oxide gel; and 
     (c) adding organic acid, water or both to the precursor of the ceramic oxide gel to peptize the metal ions; and 
     (d) heating the peptized precursor to evaporate solvent to form a thick, viscous, flexible, ductile, handleable gel that can be cast, extruded, or drawn; wherein x and y are independently 0 or 1.

This application is a divisional application based on U.S. application07/566,557, filed Aug. 13, 1990, now U.S. Pat. No. 5,402,618. Thisapplication also is related to U.S. application No. 07/325,269 filedMar. 17, 1989, which is a continuation-in-part of U.S. application No.07/155,358, filed Feb. 12, 1988, now abandoned.

The present invention is directed ceramic oxide compounds which areformable into fibers, films and other desired shapes which are useful asheating elements, fuel cell connectors, oxygen sensing elements, andother electronic applications.

BACKGROUND OF THE INVENTION

One problem with ceramic materials, typically made by sputtering, isthat the ceramic material is brittle, hard and difficult to handlewithout damaging the ceramic, and is particularly difficult to form intoa wire, fiber, or other useful forms such as spheres, bubbles, fineparticles, etc.

The present invention provides ceramic materials which can be made intovirtually any desired shape or form and which, in particular, may becast or extruded.

Other objects of the present invention will be apparent from thefollowing description of the preferred embodiments of the invention, theappended claims, and may from the practice of the invention.

SUMMARY OF THE INVENTION

The present invention provides a method for preparing ceramic materialsin a predetermined shape and form, comprising the steps of providing asolution of salts of the metals contained in the desired superconductingmixed-metal oxide of predetermined composition, wherein each of thesalts is present in an amount necessary to provide the predeterminedstoichiometric amount of each respective metal required in the desiredceramic; and wherein the counterions, or hydrolysis products thereof, ofthe metal ions for each of the salts in the solution are removable fromthe solution by evaporative methods; subjecting the solution tohydrolyzing conditions and removing the counterions and/or hydrolysisproducts thereof, and a substantial portion of the solvent, from thesolution by evaporative methods; converting the metal ions to a metaloxide precursor of the ceramic; peptizing the mixed metal oxideprecursor to form a viscous polymeric sol; forming the viscous polymericsol into a predetermined shape or form and heat-setting the sol to aflexible, ductile gel; firing the heat-set gel in the presence of oxygenat a temperature and for a period of time sufficient to oxidize andvolatilize any remaining vapors and organic materials from the gel andto form the ceramic. Novel ceramic materials according to the presentinvention are also provided, as well as novel viscous, castable,extrudable metal oxide precursors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a method for producing ceramic materialsof a predetermined shape, such as tape, fibers, and coatings, films,spheres, bubbles, etc. In the context of the description of theinvention the term sol will have its accepted technical meaning: acolloidal solution. The term gel will have its accepted technicalmeaning: a colloidal solution of a liquid in a solid. The startingmaterials for the method are soluble salts, in particular the solubleorganic salts, of the metals which comprise the final mixed metal oxidesuperconductor. The metal salts may be soluble in water, or watermiscible alcohol, mixtures thereof, or any other water-miscible solventwhich can be removed by evaporation without a reaction which isdeleterious to the formation of the ceramic. If an appropriate solublesalt of a desired metal is not readily available, but is available as aninsoluble metal halide, the metal may be incorporated, alternatively, asa colloidal gel by reacting the metal halide (such as a metal chloride)with water to make a colloidal metal hydroxide. Such a colloidal metalhydroxide may be separated from an ammonium chloride solution, thenreacted with the sol containing hydroxides and/or oxides of the othermetals to be incorporated into the ceramic.

While not intending to be limited by any particular theory, it isbelieved that the method of preparation of the ceramics of the presentinvention permit the formation of the appropriate metal oxide structureto occur on a colloidal level among particles the size of about 1-10 nmin diameter in the proper stoichiometry and lattice conformation,valence ratio and phase relationships, thereby producing compounds whichare believed to be more homogeneous than a ceramic of similarcomposition made by sputtering and sintering of mixed-metal oxide orcarbonate powders. Sputtering of metal oxide or carbonate powders allowsmixing of particles on the order of 1-10 μm in size, which are sinteredmore slowly than the colloidal particles according to the method of thepresent invention.

A solution is first prepared containing soluble salts of the metalsultimately required in the ceramic. These salts are preferably solublein water or in a water-miscible alcohol, such as methanol, ethanol,isopropanol, ethylene glycol and the like. The appropriate salts includethose which provide, as a counterion to the metal ion, an ion which isremovable by evaporative methods, or at least the hydrolysis product ofwhich is removable by evaporative methods. The salts include theorganic. counterions such as the acetates and formates, as well ascounterions which evolve as gases at an appropriate pH, such as thecarbonates, nitrates, hydroxides. To assist in solubilizing the metalsalts, polyhydroxy chelating compounds, such as, ethylene glycol, andalpha-hydroxy organic acids, such as citric acid, and the like, may beadded to form the metal salt solution. The preferred polyhydroxycompounds are citric acid and ethylene glycol. These polyhydroxycompounds and organic acids retain metal salts in solution, since somesalts would precipitate under subsequent distillation conditions.Exemplary salts of those metals which comprise the novel ceramicsinclude, but are not limited to:

    ______________________________________                                                   Soluble in                                                         ______________________________________                                        lanthanum carbonate                                                                        water (acid pH); EG/CA                                           lanthanum acetate                                                                          water                                                            strontium formate                                                                          water                                                            manganese formate                                                                          water                                                            chromium acetate                                                                           water                                                                         EG/CA = ethylene glycol and citric acid                          ______________________________________                                    

It is contemplated that in some instances an appropriate soluble salt ofa desired metal may not be readily available. In such cases an availableinsoluble metal halide, such as the metal chloride, may be used toprepare a colloidal metal hydroxide which, in turn, may be later addedto the peptized hydroxide sol containing the other metals required forthe making of the ceramic. For example, a metal chloride may be reactedwith water to form a colloidal metal hydroxide. The colloidal metalhydroxide may be separated from an ammonium chloride solution and thenadded to the sol containing the other hydroxides or oxides of the othermetals. Exemplary halide salts which may be utilized in this mannerinclude, but are not limited to:

strontium fluoride

strontium iodide

strontium bromide

lanthanum chloride

After preparation of the solution of soluble metal salts, if water isnot already present in the solution, water is then added and thesolution is subjected to hydrolyzing conditions whereby the counterionsof the metal ions, or their hydrolysis products, are converted tomoieties which are removable, by evaporative methods, such as byevolution of gas, or by evaporation of alcohols or organic acid. Thismay normally be done by distillation whereby the organic products areremoved from the metals along with a substantial portion of the organicsolvent and water. Subsequent to or simultaneous with distillation, themetals are converted by heating to oxides to form a mixed metal oxideprecursor for ceramic.

The chelates in turn are heated and polymerized to form a mass capableof holding the constituents in suspension. The preferred method is topolymerize the chelating agent, and then individually add the separateconstituents to a minimum amount of a fraction of the chelating polymer.A minimum amount of organic agents that will maintain solubility isrequired to minimize particle size and enhance fiber and film formationby limiting the amount of by products produced on curing.

The metal oxide precursor, which is then typically a homogeneoussemi-solid, is peptized to a sol, or fluid colloidal system, usually byaddition of an organic acid, such as acetic acid, and/or water. Thispeptization step is usually conducted by heating at a temperature ofless than about 100° C. At this time, metal colloidal gel, prepared byreacting metal halide and water, may be added to provide the metal ormetals for which there were no available soluble salts. During thispeptization process, the polymeric chains of the inorganic oxides arethen formed.

Heating this sol produces a thick, viscous gel which can then be castinto thin strips, extruded, or drawn, as continuous or discontinuousfibers, into thin monofilamentary fibers or multifilamentary tows.

The gel can also be diluted and sprayed as a chemically homogeneouscoating, for example, on a resonance cavity of a particle accelerator.Upon forming the gel into its desired form either as continuous fibers,discontinuous fibers, tape, coating, or otherwise, the gel is heat-set,usually by contact with a hot flowing air environment, typically atabout 80°-120° C. The resultant hard-gelled oxide is ductile, flexibleand handleable, and thus is an improvement over products made by thesintered powder method.

Various sintering aids and glass formers may be added at this time inthe ratio of 0.1 to 10 wt. %. Typical glass formers include P₂ O₅, PbOand B₂ O₃. Other sintering aids may include reinforcements such assilicon nitride, silicon carbide and similar fibers. These aids improveflexibility, toughness and processability after sintering. The gel isthen further advanced to the desired viscosity by heating with orwithout vacuum. If heated with vacuum, the vacuum and heat should beapplied slowly.

As a final step, the mixed metal oxide in its desired hard-gelled shape,is fired at a temperature and for a period of time sufficient to oxidizeand volatilize any remaining vapors and organic materials, therebyleaving an intact, dense, mixed metal oxide ceramic its desired form.While this period of time will vary, usually one to six hours willsuffice. Usually, the firing temperature will be in the range of about600° C. to 900° C. The preferred firing temperatures are in the range of850° C. to 900° C., most preferably at about 875° C. Further sinteringat temperatures up to 1450° C. may be desirable, depending on the natureof the sintering aids, if any are used, and the size and shape of thefinal ceramic product.

The final ceramic materials produced according to the present inventioninclude, but are not limited to, those having the formula La_(1-x)Sr_(x) Cr_(1-y) Mn_(y) O₃ wherein x and y are independently 0 or 1.

Having described the preferred embodiments, the following examples arepresent by way of illustration, but are not intended to limit theinvention in any way.

EXAMPLE

About 0.158 moles of lanthanum acetate, 200 ml of acetic acid and 500 mlof water were combined in a 1 liter flask. The slurry was heated tosolution on a rotary evaporator in a water bath held at 60° C.Meanwhile, 30 grams of citric acid and 54 grams of ethylene glycol weremixed in a beaker. This mixture was heated to 155° C. while stirring,and the temperature was maintained for 20 minutes. Then the solution wascooled and 150 ml of water were added. The solution was transferred tothe flask on the rotary evaporator and the container was washed with anadditional 50 ml of water. Next, the water bath temperature was broughtto 60° C. and solvent was removed under vacuum. Meanwhile, 0.175 molesof manganese formate with 200 ml of acetic acid were heated to reflux.Then 300 ml of water were added and heating continued at 90° C. untilthe salt was dissolved. This solution was cooled to 65° C. and added tothe 1 liter vessel. Solvent was removed while repeating the procedurewith 0.0175 moles of strontium formate, 10 ml of acetic acid and 25 mlof water. After adding the last solution, solvent was removed by raisingthe bath temperature to 80° C. Films, fibers, and the like may beprocessed directly from the transparent amber sol.

Having described the preferred embodiments above of the invention, othermodifications and alternative embodiments will be apparent which will bewithin the spirit and scope of the present invention. The invention isnot intended to be limited except by the scope of the following claims.

We claim:
 1. A La_(1-x) Sr_(x) Cr_(1-y) Mn_(y) O₃ ceramic oxide gelobtainable by the process comprising the steps of:(a) providing acolloidal solution of polyhydroxyorganic chelating agents selected fromthe group consisting of ethylene glycol and citric acid, and metal saltsselected from the group consisting of lanthanum, strontium, chromium,and manganese, each salt being present in an amount necessary to providethe stoichiometric amount of metal ions required in the ceramic oxidegel; (b) heating the colloidal solution to hydrolyze and polymerize themetal ions in the solution to a mixed metal oxide precursor of theceramic oxide gel; and (c) adding organic acid, water or both to theprecursor of the ceramic oxide gel to peptize the metal ions; and (d)heating the peptized precursor to evaporate solvent to form a thick,viscous, flexible, ductile, handleable gel that can be cast, extruded,or drawn; wherein x and y are independently 0 or
 1. 2. The ceramic oxidegel of claim 1 further comprising 0.1-10 wt % of a glass former.
 3. Aceramic oxide gel of claim 2 wherein the glass former is selected fromthe group consisting of P₂ O₅, PbO and B₂ O₃.
 4. The ceramic oxide gelof claim 1 further comprising 0.1-10 wt % of a sintering aid.
 5. Theceramic oxide gel of claim 4 wherein the sintering aid is selected fromthe group consisting of silicon nitride and silicon carbide.
 6. Theceramic oxide gel of claim 5 further comprising 0.1-10 wt % of a glassformer selected from the group consisting of P₂ O₅, PbO, and B₂ O₃.
 7. Aceramic oxide precursor colloidal solution for making a La_(1-x) Sr_(x)Cr_(1-y) Mn_(y) O₃ ceramic oxide, wherein x and y are independently 0 or1, and wherein the mixture solubilizes the metal salts comprising astoichiometric mixture of soluble metal ions and counterions oflanthanum, strontium, chromium, and manganese metal salts in a mixturehaving an effective amount of a chelating agent selected from the groupconsisting of ethylene glycol and citric acid or mixtures thereof tomaintain solubility of the metal ions, and an effective amount of apeptizing agent selected from the group consisting of organic acids,water, or mixtures thereof, wherein the counterions of each metal ion orthe hydrolysis products of the counterions in the mixture aredecomposable and wherein the peptizing agent is present in the minimumamount to maintain solubility of the metal ions by peptizing the metalions to a colloidal solution and to minimize the particle size so as toenhance fiber and film formation.
 8. The ceramic oxide precursorcolloidal solution of claim 7 wherein the peptizing agent includesacetic acid.
 9. The ceramic oxide precursor colloidal solution of claim8 wherein the lanthanum salt is lanthanum acetate, wherein the manganesesalt is manganese formate, and wherein the strontium salt is strontiumformate.
 10. The ceramic oxide precursor colloidal solution of claim 8wherein the chromium salt is chromium acetate.
 11. The ceramic oxideprecursor colloidal solution of claim 8 further comprising 0.1-10 wt %of a glass former and, optionally, 0.1-10 wt % of a sintering aidselected from the group consisting of silicon nitride and siliconcarbide.
 12. The precursor colloidal solution of claim 7 wherein thecounterions of the metal salts are selected from the group consisting ofacetates, formates, carbonates, nitrates, hydroxides, or mixturesthereof.